1. Field of the Invention
This invention relates to gemstones and, more particularly, to multiplet gemstones, such as doublets and triplets, which incorporate pleochronic crystals.
2. Description of the Prior Art
Many of the well-known gem minerals are pleochroic. That is, a single piece of such a material exhibits various colors, depending on the direction of viewing and/or the polarization direction of the illuminating light. The fundamental effect involves the polarization direction of the light. The viewing angle dependence in unpolarized light arises from the elimination of light polarized parallel to the viewing direction, since light waves are purely transverse excitations. These effects arise in non-cubic, optically uniaxial or biaxial crystals from the influence of an anisotropic host crystal structure on the transitions between electron energy levels of the impurity ions which give rise to the color.
Well-known gem crystals which exhibit pleochroism to a greater or lesser extent include alexandrite, andalusite, axinite, beryl, chrysoberyl, cordierite, dichroite, emerald, epidote, kyanite, periodot, ruby, sinhalite, spodumene, tourmaline and zoisite.
In the course of cutting crystals of these minerals to obtain faceted gem stones, it is well-known that often one must control the crystallographic orientation of the stone to obtain the desired color in the finished gemstone, or alternatively, that the color of the finished gem may be varied to some extent by varying the crystallographic orientation. This is particularly important, for example, in the faceting of certain types of tourmaline. Such a stone cut with the table (top) facet parallel to the c plane may appear nearly opaque or black, while the same stone cut with the table facet perpendicular to the c plane exhibits a desirable blue or green color. Similarly, certain cordierites may appear nearly colorless in certain orientations and deep blue in others.
Multiplet gemstones have been used in the past for various purposes. For example, D'Esposito in U.S. Pat. No. 1,745,607, issued Feb. 4, 1930, describes doublet stones in which two components of natural beryl are cemented together with a transparent cement incorporating an appropriate coloring agent to produce a composite stone exhibiting the color of emerald. In this way, the refractive properties of beryl, which resemble those of emerald, are combined with the color of the intervening colored cement. However, if an originally pale or colorless beryl is formed into a colored composite gem by means of a colored cement, the resultant composite is pleochroic only to the extent that the original stone was colored, D'Esposito's contrary implication notwithstanding.
Other attempts to alter or control the optical properties of gemstones have been employed. For example, Boone, in U.S. Pat. Nos. 2,663,171 and 2,699,706, issued Dec. 22, 1953 and Jan. 18, 1955, respectively, discloses a variety of combinations of birefringent, polarizing and reflecting layers over and/or under transparent supporting elements. These layers provide rainbow-like interference colors, which Boone refers to in his claims as "variegated." The choice of angle (e.g., 45.degree.) between polarizing and birefringent directions affects the extent of the rainbow effect. The support element generally makes no direct contribution to the color, although Boone does disclose that a dye could be incorporated in the supporting element to modify the interference colors. None of Boone's embodiments provides a gemstone with a "pure color"; i.e., transmission in a narrow wavelength band.
In highly doped natural or synthetic alexandrite, the daylight green color is often obscured by red overtones, especially in thicker sections. Cline et al. in U.S. Pat. No. 3,912,521, issued Oct. 14, 1975, disclose addition of iron as an impurity as a means of improving the daylight green coloration of highly doped synthetic crystals of larger size. However, we have observed that this method is not totally effective.